I don't know that you can really protect yourself with a dB meter and a pair of headphones. The volume you're getting is not dBs as far as the room is concerned, but perceived dBs based on the equivalent when you jam the drivers up to your ear. When headphone companies give you such stats, they're repeating the stats reported by the manufacturer of the driver. Unless they've done separate tests on their own, they're just passing the info down the line, even though the driver's specifics will be affected by how they're housed, the tips or cushions used, et cetera.
On the Homemade IEMs board, there have been questions about the effect of using multiple drivers, because more drivers means - among other things - the ability to get maximum volume out of the same number of milliwatts. Some have reasoned that this could get hazardous to hearing, but it's only hazardous if you turn the volume up beyond what's reasonable. I would argue that it's good to have a driver that can do more than it needs to because you don't want to press a driver to its limits. It's not good for the driver and it's not good for sound. It's also not good to rev your car's engine to the max or run your iPod as high as the volume control will go.
This is what I do. When I put a song on, I treat myself. I enjoy the sound at a luxuriously rich volume (high without being ridiculously high). But as the song progresses, or as I move on to the next song, I cut back a little - not so much that I lose enjoyment but a little now that I've settled into the music. I find I can do this in several stages and that, in time, I've learned to listen to the music and not merely react to the visceral pleasure of enjoying an extreme.
It's human nature to "do it, do it, do it till you're satisfied" (whatever it is). But you don't need to. Once you get settled, you can cut it back, little by little until you're not going for excess. You're just wine-sipping that sound and loving it.
My advice - for what it's worth - is to cultivate your own sense of reasonable volume. If and when you do that, you'll notice the situations - apart from headphone sessions - where it's just too loud. Hearing isn't like a tan, where you're trying to develop some resistance, some tanning of the ear canal. That's called going deaf. If you're sensitive to sound, protect that sensitivity by protecting your ears. It's good to know when to duck out of a situation, or to have some kind of protection with you when you can't. But for listening, there's no need to be either a prude or a fool. Just cultivate a discernment that allows you to distinguish good volume from volume-for-effects. It's the latter that will damage your ears.
